The invention relates to a method for determining a position of a movable part of a coordinate measuring machine and a coordinate measuring machine.
The mechanical and kinematic construction of the coordinate measuring machine (abbreviated CMM below) may differ. By way of example, this may relate to a CMM with a portal design, articulated arm design, parallel kinematic design or horizontal arm design. In all cases, the CMM has a movable part, at which a measuring sensor is fastened or coupled either directly or indirectly. Therefore, the measuring sensor is also moved by moving the movable part, in particular to bring said measuring sensor into a position and/or alignment in which a test object is measured with the aid of the sensor and in order to determine coordinates of the test object. Alternatively, or additionally, the measuring sensor can sense the test object during the movement, for example in a scanning manner. The movement of the movable part is driven by at least one drive of the CMM.
By way of example, the sensor is a measuring head mounted at a movable part (for example a quill or an arm) of the CMM. On the measuring head it is possible to mount a probe (e.g. a probe pin), in particular, using which the CMM probes the surface of the workpiece in a tactile manner. Therefore, in particular, a probe for the tactile probing of the workpiece to be measured is also an example of a sensor or of a part of the sensor. The measuring head has a sensor system, in particular, which generates measurement signals whose evaluation enables the coordinates to be determined.
However, other measuring sensors also crop up in coordinate measuring technology. By way of example, the sensor may merely initiate the measurement of the coordinates. This is the case for example for a switching measuring head which produces a switching signal upon contact with the workpiece to be measured, which switching signal initiates the measurement of the coordinates e.g. by reading off the scale or the scales of the position measuring system at the movable part or the movable parts of the CMM. In principle, the sensors can be classified into sensors that carry out measurement by contact (tactile probing of the workpiece) and sensors that do not carry out measurement by contact. By way of example, optical sensors (e.g. cameras) or capacitive sensors for coordinate measurement are sensors which are not based on the principle of tactile sensing. Moreover, it is possible to classify sensors according to the type or size of the in particular simultaneously detected region of the workpiece. In particular, sensors may measure coordinates just of a point or of an area on the surface or else in the interior of the workpiece or measure coordinates of a volume of the workpiece.
In particular, the measuring sensor can be securely connected to the movable part or to at least one of the movable parts of the coordinate measuring machine, the position of which is measured by a position measuring system of the CMM during the operation of the coordinate measuring machine. A secure connection should also be understood to mean a connection that is detachable again, said connection for example being establishable by, or consisting of, an interchange interface. As an alternative or in addition to a secure connection, there may be a movable connection, or said movable connection may be established, between the measuring sensor and at least one of the movable parts of the coordinate measuring machine. By way of example, the measuring sensor may be connected to one of the movable parts via a rotary joint with one axis of rotation or with a plurality of axes of rotation. By way of example, this movable part may be the quill of a coordinate measuring machine with a portal design or gantry design, or the horizontal arm of a coordinate measuring machine with a horizontal arm design. At least one interchange interface may also be used in the case of the movable connection. The respective interchange interface permits coupling and decoupling of the measuring sensor and/or of the rotary joint.
In particular, the movable part may be an elongate part, e.g. an arm, wherein the length of the elongate part is a multiple (e.g. at least five times or at least 10 times) of the width and depth of the elongate part. The width and depth may be measured in two mutually perpendicular directions that are perpendicular to the longitudinal direction.
Mechanical vibrations which, in particular, are excited by the at least one drive of the CMM may occur in the CMM of the aforementioned type with at least one movable part and a measuring sensor that is directly or indirectly connected or coupled to the movable part. Such vibrations also occur at the movable part to which the sensor is indirectly or directly connected or coupled. Although the position measuring system of the CMM measures the current position of the movable part, it does not, in almost all movement positions, measure the position of the region of the movable part that is connected or coupled to the measuring sensor. In almost all movement positions, the point at which the position measuring system measures the position of the movable part lies at a significant or even far distance from the region to which the measuring sensor is coupled or connected. Therefore, the position measuring system is unable to directly measure the unintended changes in the position of the region on account of vibrations.
On the other hand, the position of the region to which the measuring sensor is coupled or to which it is fastened is required for determining the position of the measuring sensor and hence also for determining the coordinates of the test object that is measured with the aid of the measuring sensor. The coordinates should be determined not only in the coordinate system of the measuring sensor but, in particular, in the coordinate system of the test object or a stationary base of the coordinate measuring machine.
By way of example, in the case of a coordinate measuring machine with a portal design (i.e. a bridge design), vibrations may occur at the lower end of the quill to which the measuring head is coupled, in the direction (denoted Y-direction as a rule) in which the entire bridge or the entire portal is displaceable. In the case of the coordinate measuring machine with a horizontal arm design, significant vibrations may occur in the direction (denoted X-direction as a rule) in which the horizontal arm is movable together with the stand that is displaceable relative to the base, in particular in the case of a greatly extended horizontal arm to which the measuring sensor is attached.
It is possible to provide an acceleration sensor, for example at the lower end of the quill or at the end of the horizontal arm, said acceleration sensor measuring the accelerations on account of vibrations in the region to which the measuring sensor is coupled or to which it is connected. The measurement values of the acceleration sensor depending on time can be integrated twice over time such that the position is obtained as a function of the location. However, this assumes that the initial value for integrating twice is known and the acceleration sensor and also integrating twice over time do not produce a significant error that falsifies the result. Trials by the inventors have yielded that, in any case, acceleration sensors which can be assembled on a CMM for permanent operation at justifiable costs do not supply sufficiently accurate determination results for the position over typical operation times of coordinate measuring machines if the effects of mechanical vibrations should also be considered. Instead, it was found that, over time, the phase and amplitude of the determination result increasingly deviate from the actual phase and amplitude of the vibration.
It is an object of the present invention to specify a method for determining a position of a movable part of a coordinate measuring machine and a coordinate measuring machine which facilitate determining the position of the movable part, even if mechanical vibrations occur at the movable part.